700-21631
APPLICATION NOTE

VHF DIGITAL CLOCK USING CROVEN CRYSTALS MODEL
CC1066

by Charles Wenzel

VHF crystal oscillators
typically employ 3rd or 5th overtone crystals
for superior stability and jitter performance.
Since the crystal will readily oscillate at the
fundamental or lower odd overtones, some method
of selecting the desired mode is needed. The
following circuit uses a simple grounded-base
configuration with a tuned circuit in the
collector to select the desired mode and to
provide feedback for oscillation. The 0.1 uH
resonates with the trimmer capacitor and the
series combination of the 15 pF and 39 pF at the
desired frequency, 100 MHz in this case. The 15
pF and 39 pF divide the collector voltage and
step the current up sufficiently to reliably
start oscillation. Oscillation builds until the
1N5711 diodes begin to conduct, loading the
collector sufficiently to limit the oscillation
level. This type of limiting keeps the
transistor in a linear region. The lower voltage
across the 39 pF is sufficient to drive a
self-biased AC logic inverter.

The circuit loads the
crystal with about 15 pF but the crystal used in the prototype
was designed for series resonance. The 0.22 uH in series with
the crystal easily brings the frequency down and the value may
be varied to move the tuning window up or down. The choke could
be eliminated by specifying the CC1066 to tune with a 15 pF
load. In the event the frequency window needs to be raised, the
inductor could be replaced with a capacitor. The tuning range of
the 9-35 pF trimmer will be greater with series inductance,
however. The range of the prototype is +- 18 ppm with the 0.22
uH, +-12 ppm with a jumper, and only +-9 ppm with a series 22
pF. By selecting a series-resonant crystal and choosing an
inductor to add in series to bring the frequency back down, a
wider tuning range results, giving the circuit more tolerance to
component variation.

The trimmers could be
eliminated for application not requiring precise
frequency. The collector inductor should be a
precision type and the collector trimmer would
be replaced with a combination of two fixed
capacitors selected to peak the oscillation. The
precise value will be a function of the board
layout and transistor type, and using two
capacitors makes it easier to hit a precise
value. The crystal trimmer
and 0.22 uH inductor could be replaced with
jumpers if the crystal is specified for a load
near 40 pF. The crystal could be specified for a
load capacitance of 22 pF and a fixed capacitor
could be added in series, selected to bring the
frequency close to the desired nominal value.
That fixed value will be near 47 pF, somewhat
dependent on the layout and component choices.

The circuit is fairly
forgiving to component variation. The transistor
isn't critical; even the relatively slow 2N4401
worked well in the prototype. The 180 ohm in
series with the diodes may be increased for
higher drive level and a larger signal at the
input of the AC gate. Other fast logic types
with high input impedance like UHS Tiny Logic
may also be used. The 2.2 uH may be replaced
with a jumper with minimal loss in performance.
For the best noise floor, leave it in and
increase the 180 ohm to 470 ohm to drive the
logic with faster edges. AC logic has robust
input protection diodes but other logic families
do not, so make sure to not set the amplitude
too high. The floor will vary
from -140 to -150 dBc at the output of the gate
as a function of the oscillation level. The
noise at the input of the gate is about -152 dBc,
improving to about -158 dBc with a quality low
noise RF transistor and higher signal levels. That voltage may be
buffered with a high speed op-amp for sine wave
output.